Allowable surface pressure

[mar2805]

Hi!
I neeh help guys.
The pressure formula p=F/A
Lets say that Im standing on a surface that has value of allowable pressure that it can stand of 10kN/m2
My weight is 100kg, wich is equal to 1kN.
Im standing on one foot.
Lets say that the area of my feet thats touching teh graound around 0.3m x 0.05m, wich gives a surface od 0,015m2.
The pressure that Im generating on a surface is 1kn/0,015m2=66,66kN/m2
Is it ok to say that the ground surface that Im standing on, will fail! Since 66,66kN/m2 > 10kN/m2
Lets imagine another situation where you have the same ground surface with allowable pressure of 10kN/m2.
Then I load it with a concrete box wich has dimensions 1,00 x 1,00 x 0,4m (height). Since the density of the concrete is around 25Kn/m3 it will generate a force of F=a x b x c x density wich equals= 1 x 1 x 0,4 x 25=10KN.
If I divide this force with the area that the box is touching the ground I will get an pressure value of 10kN/m2
We can say that the critical value is acchived a that the ground surface will probably fail.
The problem with this is that I cannot simpy visual this.
Me weight Vs. the concrete block
I generate greater pressure on a surface then the concrete block wich is 0,4m thick!
The ground surface will fail faster under my load and not under this concrete block!
?!)"?!
Can someone confirm this?
Thanx!

 

[Ron]

An ultimate bearing capacity has a range of areas for which it is valid. As the area approaches 0, so does the bearing capacity. For your soil type, an area of 1m^2 might have a significantly greater ultimate bearing capacity than say an area of 0.1m^2. Stress modes in the soil change, overburden confinement becomes more important, and the value becomes less predictable. Keep in mind also, that the bearing capacity given by the Geotech is not an ultimate capacity...it usually has a safety factor tacked on.

If you think of walking on a soil surface and seeing your heel prints behind you, each of those represents a bearing capacity failure...small, but nonetheless a failure.

 

[mar2805]

Well, to be ohnest Im talking aboth thermal insulation that you put on the foor. Styropor (EPS).
The reason Im asking all this is because people are puting class that has load bearing capacity of 100kN/m2, for residental objects!
I found that to be reaaaaaaly to much. I talked to the selling stuff on this matter. Their answer was that styropor below 100kN/m2 is considered soft!?!
Must be somekind of selling propaganda! I mean 100kN/m2 give me a break! Maybe for garages where you ahve traffic load of cars, but for home use, no way!

 

[Ron]

Your original statement should have included the fact that you were considering EPS, not soil.

Most medium to high density EPS materials will have a compressive strength of around 100 to 175kN/m^2. Your consideration is on the low end of that range and should not be an issue, provided the material is fully supported.

 

[mar2805]

But do you agree that 100kN/m2 is maybe to much for residental use?
Im asking this couse you have EPS with compressive strenght od 10kN/m2 these days wich are use to damphen the walking noise thru structure.

 

[Ron]

If you are using it below the concrete, then no, it is not too much.

Don't confuse a "structural" application with a non-structural application.

 

[mar2805]

I dont get you here...Can you clarify a bit.
Thanx

 

[Ron]

What exactly is your application for the EPS? Are you using it below a concrete slab for insulation or are you using it for some other purpose? Is it roof insulation? Is it wall insulation? Is it being used as a sound mitigation layer? Will there be materials applied over the EPS and if so, what are they? What is the thickness and why are you concerned about the compressive strength of the EPS.

You have given almost no information so that we can help you with this. Please lay out your application and perhaps we can give you some suggestions, comments or advice that will be helpful. Otherwise, we are just trading random comments that resolve nothing.

 

[mar2805]

It will be used as a thermal insulation placed directly on the co ncrete slab.
The thicknes of the EPS will be 4cm.
Then I will have a layer of concrete of 5cm over it and a layer of glazing (tiles) of 1 cm above all + load from house equipment, people load....

 

[Ron]

With such a thin topping slab, I would use a medium to high density EPS. It should have a reasonably high compressive strenth and should be anchored (adhered) to the underlying slab prior to placement of the topping.

The EPS you have questioned would be on the lower side of the range that I would specify.

 

[mar2805]

Thanx man!